Conventionally, two types of methods have been known for developing an electrostatic latent image in electrophotography and electrostatic printing. One method is a two-component-type system wherein colored resin powder called "toner", and a carrier composed of iron powder, glass beads and the like are rubbed together to make the toner charged; then the charged toner is electrostatically attracted on an electrostatic latent image of a photosensitive member so as to develop the image. The other method is a one-component-type system wherein a carrier is not used; instead, a toner obtained by means of melting and kneading the following materials, such as binder resin, coloring agent, and magnetic powder, is used for developing a image. In both methods, a fixing process whereby a toner is transferred and fixed on a copy paper, is indispensable. There are two types of fixing methods; one is a heat-roller-fixing method wherein an image is melted and fixed by a heat roller; the other is a pressure-fixing method wherein an image is fixed by pressure.
In recent years, energy saving in both fixing methods is strongly demanded, i.e., improvement in low-temperature performance for the heat-roller-fixing method, and improvement in low-pressure performance for the pressure-fixing method are required. To cope with the above-described demands from the aspect of a toner design, a glass-transition temperature of the binder resin can be lowered in order to achieve low-temperature fixability; however, on the other hand, the use of the resin having a low glass-transition temperature causes the agglomeration of the toner. Consequently, the toner forms an agglomerate during storage, or electric charge quantity decreases due to the lowered flowability in the developing machine.
As for the pressure-fixing method, various methods have been proposed, such as Japanese Patent Publication Open to Public (hereinafter referred to as Japanese Patent O.P.I Publication), No. 14260/1982, No. 146261/1982, No. 41648/1982, and No. 44155/1982. According to the proposed methods, a toner is obtained by melting and dispersing magnetic powder in wax followed by grinding and classifying into fine particles, and then dispersing the fine particles in a liquid solution which has dissolved a resin, such as polystyrene, followed by spray-drying. According to this method, however, the particles stick together because of softening the surface resin thereof when spray-dried. Even if the size of the core material may be made the same, the particle-size distribution becomes large because of the sticking and the charge quantity fluctuates, thus causing deteriorated image quality.
Also, as described in Japanese Patent O.P.I. Publication No. 25156/1986, a capsule toner can be obtained through the following procedures; First, porous polymer particles are dispersed in an aqueous medium containing metal salt mainly composed of iron salt to precipitate metal hydroxides or oxides on the polymer particles (core particles) and then the precipitated polymer particles are used as seed particles to polymerize in an aqueous solution containing a polymerizable monomer to form the capsule toner. According to this method, however, the metal oxide layer formed on the core particles is formed only by precipitation. Therefore, the layer thickness of the metal oxide is limited, and the quantity of the magnetic material present on the surface of polymer particles depends on the surface area of the particles. Accordingly, in order to increase the surface area, the polymer particles have to be made porous. However, to make the polymer particles porous, extremely large amount of a crosslinking agent must be used. Consequently, it is impossible to provide the heat-roller fixability or pressure fixability to the polymer particles. Therefore, in order to provide fixability to the toner, it is required to further form a resin layer on the surface of the polymer particles (core particles). However, in order to ensure low-temperature fixability or low-pressure fixability, glass-transition temperature of the further formed resin layer on the surface of the particles have to be lowered, whereby blocking resistance is deteriorated.